This application claims the priority of Japanese Patent Applications No. H9-105490 filed on Apr. 23, 1997, H9-106975 filed on Apr. 24, 1997, H9-107141 filed on Apr. 24, 1997, H9-257542 filed on Sep. 5, 1997 and H9-362693 filed on Dec. 12, 1997, and Japanese Patent Application filed on Mar. 30, 1998 with the title of the invention of "Resistor-Incorporated Spark Plug, Resistor Composition for Spark Plug and Manufacturing Method of Resistor-Incorporated Spark Plug", which are incorporated herein by reference.
The present invention relates to spark plugs to be used for internal combustion engines and, more particularly, to a spark plug into which a resistor for prevention of occurrence of radio frequency noise is incorporated and the manufacturing method thereof.
As this type of spark plug, there has conventionally been known one having a structure that a terminal is fixed in one end portion of a through hole formed along the axial direction of an insulator while a center electrode is similarly fixed in the other end portion of the through hole, where a resistor is placed between the terminal and the center electrode within the through hole. This resistor is implemented by one which is formed through steps of mixing amorphous carbon (e.g., carbon black) into glass powder and/or dielectric ceramic powder and thereafter sintering the mixture by hot press or the like as shown in Japanese Patent Laid-Open Publication S61-104580, S61-253786, or H2-126584.
In this connection, recently internal combustion engines such as automobile engines are on the trend toward higher output, while power supply ability has been on the increase for improvement of ignitionability. Also, with the downsizing of internal combustion engines, resistor-incorporated spark plugs have also been required to be smaller in size and higher in performance. Under these circumstances, there is an issue that when some high load is applied on such a resistor-incorporated spark plug, particularly on a small-size spark plug with a small-diameter resistor, the carbon that imparts electrical conductivity to the resistor would burn, causing the resistance value to increase, so that a stable load life characteristic could not be obtained.